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Statement - The water pouch of instant...

Statement - The water pouch of instant cold pack for treating
athletic injuries breakes when squeezed and `NH_(4)NO_(3)` dissolves
lowering the temperature.
Explanation - Addition of non-volatile solute into solvent results into depression of freezing point of solvent.

A

if both Assertion and Reason are true and reason is the correct explanation of the assertion.

B

if both assertion and reason are true but reason is not the correct expalanation of the assertion

C

if assertion is true, reason is false

D

both assertion and reason are false statements.

Text Solution

Verified by Experts

The correct Answer is:
a
Frezzing point of a substance is defined as the temperature at which the vapour pressure of its liquid is equal, to the vapour pressure of the corresponding solid. Since the addition of a non-volatile solute always lowers the vapour pressure fo solvent. therefore it will be in equilibrium with solid phase at a lower pressure and hence at a lower temperature.
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Why is freezing point depressed when a non-volatile solute is added to a solvent ?

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A system of greater disorder of molecules is more probable. The disorder of molecules is reflected by the entropy of the system. A liquid vapourizes to form a more disordered gas. When a solute is present, there is additional contribution to the entropy of the liquid due to increased randomness. As the entropy of solution is higher than that of pure liquid, there is weaker tendency to form the gas. Thus, a solute (non-volatile) lowers the vapour pressure of a liquid, and hence a higher boiling point of the solution. Similarly, the greater randomness of the solution opposes the tendercy to freeze. In consequence, a lower temperature must be reached for achieving the equilibrium between the solid (frozen solvent) and the solution. The elevation in boiling point (DeltaT_(b)) and depression in freezing point (DeltaT_(f)) of a solution are the colligative properties which depend only on the concentration of particles of the solute and not their identity. For dilute solutions, (DeltaT_(b)) and (DeltaT_(f)) are proportional to the molarity of the solute in the solution. A mixture of two immiscible liquids at a constant pressure of 1.0 atm boils at temperature

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